Effect of material damage on the spallation threshold of single crystal copper: a molecular dynamics study

High velocity impact of copper plates using molecular dynamics has been performed to study the spallation of single crystal copper at impact velocities of 1100 and 1000 m s(-1). The molecular dynamics code LAMMPS (Large-Scale Atomic/Molecular Massively Parallel Simulator) with the embedded atom method potential is used for this study. It is found that for an impact velocity of 1100 m s(-1), nucleation and growth of multiple voids take place which lead to the spallation of the material. For the impact at 1000 m s(-1) in the < 1 0 0 > impact direction, the material does not undergo spallation but gives a spall-like signal in the free surface velocity of the target. We show that the tension developed by first traversal of the shock wave creates various kinds of defects in the target. These become void nucleation sites during the subsequent traversal of the shock wave. The presence of void nucleation sites due to the first traversal of the shock leads to the nucleation of the voids at a lower tensile pressure. We also show that the spall-like signal in the free surface velocity of the target at 1000 m s(-1) impact along the < 1 0 0 > direction occurs due to stress relaxation resulting from the nucleation and growth of the voids without physical separation of scab from the target.